I find strange the idea that the characters are to be introduced by a movie which, by its end, wipes out the timeline they're in. You'd be effectively wiping out the characters themselves - you can make a movie about the characters in a different timeline, but they will not be the same characters you've shown in DoFP and whatever happened to them in this movie won't matter. There'd be no connection or progression whatsoever.

It's like as if they made the last Star Trek reboot a movie in which all the characters die at the end, and then in the last shot we see their alternate versions and the movie goes, well never mind all that you've just seen, here are the real adventures of Kirk and Spock and the rest.

Well, it could create an alternate timeline, meaning the original timeline is still intact; or it alters the timeline so the dystopian future never happens. That altered timeline could be the original trilogy, but even that doesn't end well - Xavier explodes, Rogue is cured, Cyclops dies, Jean dies.

It all depends what the studio wants to do next and, if it's another film with the original cast, whether those actors are keen to come back for more.

But if people want more of Alan Cumming's Nightcrawler, Famke's Jean/Phoenix and James's Cyclops, and the studio is keen to do that, then somehow the movie's timeline twists have to allow for their return.

__________________Show me an X-Men comic where Mystique is the leader and walks round all day as a bored blonde